Fruits: dirty-white cypselas, each one with a pappus of 4-5 mm; (open) capitula with 8-15 mm diameter.

Flowering: June to August.

Similar species

The several Conyza species in Portugal are very similar and hard to distinguish. Additionally, the Conyza species hybridize very rapidly among themselves, originating hybrids with intermediate characteristics, causing further difficulty in the identification.

Conyza bonariensis is distinguished from the other species of Conyza (Conyza canadensis, Canadian fleabane, andConyza sumatrensis, tall fleabane) for having a smaller plant syze and for having bigger and denser capitula (on the fruit) with dirty-white cypselas. It is also the only species that presents the shorter main axis overtopped by the lateral branches.

Inflorescences: comparison of the three most common Conyza species in Portugal.

Characteristics that aid invasion

It propagates by seed, producing high amounts of seeds (2000 to 230000 seeds per plant) of reduced persistence (2 to 3 years) and a graduated germination. The seeds are efficiently dispersed by the wind, rapidly enhancing their distribution area.

A ruderalweed species,very frequent in disturbed areas: urban areas, roadsides and crop fields or common lands. It also appears in natural and semi-natural areas (e.g., dunes) normally associated to disturbance events.

Impacts on ecossystems

It forms dense areas that prevent the growth of native vegetation.

Economic impacts

High costs in applying control methodologies, mainly in crop areas.

It reduces productivity in crop fields.

Other impacts

Due to the high production of pollen, it is considered an allergenic plant.

Controlling an invasive species demands a well-planned management, which includes the determination of the invaded area, identifying the causes of invasion, assessing the impacts, defining the intervention priorities, selecting the adequate control methodologies and their application. Afterwards it is fundamental to monitor the efficiency of the methodologies and recuperation of the intervened area as to perform, whenever necessary, the follow-up control.

The control methodologies used for Conyza bonariensis include:

Physical control (preferencial methodology)

Hand pulling: it is applied to plants of all sizes. In more compacted substrates, hand pulling should be made during the rainy to facilitate the removal of the root system.

Agricultural practices of soil mobilization (tilling, harrowing): it is applied to plants of all sizes when dense infestations occur. It should be done before flowering.

Chemical control

Foliar application of herbicide. Spray with herbicide (active substance: glyphosate) limiting as much as possible its application to the target species. The herbicide application should be done during the early stages of the plant’s development, before the primary axis is developed.

The continued use of this herbicide has recently originated the appearance of several biotopes of Conzya that are resistant to it. However, as long as properly used, the herbicide application constitutes an effective way to reduce invasion.

Visit the webpage How to Control for additional and more detailed information about the correct application of these methodologies.

Invasive Plants

Many ecosystems in Portugal are deeply affected by invasive species, either animal or plants. This means considerable economical losses, huge impacts on biodiversity, changes in the ecosystems services and even public health issues.

Our goal is to explain this problem, educating not only the common citizen but also specialized technicians to recognize and control invasive species.